A thin film of a I-III-IV.sub.2 type compound whose band gap ranges from 1.0 to 1.8 eV, such as CuInSe.sub.2, is used as an absorber layer in solar cells. Multi-source evaporation and selenization are known as methods for forming a CuInSe.sub.2 thin film (cf. A. Rockett and R. W. Birkmire, J. Appl. Phys. Vol.70, No.7, p.81). In the multi-source evaporation, a CuInSe.sub.2 film is formed on a heated substrate by evaporating the elements (Cu, In, and Se) from the respective evaporation sources. Several well-known evaporation processes, which provide highly efficient solar cells, include a process for forming a Cu--In--Se film in a Cu-rich composition (Cu/In&gt;1). The reason is as follows. If a thin film is formed on a substrate at a temperature of at least 500.degree. C. in a Cu-rich condition for the composition, Cu--Se based impurities are deposited as well as the CuInSe.sub.2. The Cu--Se based impurities function as a flux, and thus the crystal particles of the CuInSe.sub.2 grow bigger. In another case, the band gap (Eg) of the CuInSe.sub.2 (absorber layer) is made closer to the best value for absorption of sun light (1.4 eV), so that the efficiency of the solar cell is improved. In general, the band gap of the absorber layer is increased by dissolving CuGaSe.sub.2 (Eg=1.68 eV) in the CuInSe.sub.2 (Eg=1.0 eV).
When Cu(In.sub.1-X Ga.sub.X)Se.sub.2 solid solution is used for an absorber layer, the band gap will be enlarged as the quantity of Ga solid solution (X) rises. The conversion efficiency of the solar cell, however, does not increase so easily even if the solid solution thin film is used for the absorber layer. The conversion efficiency is maximum when the quantity (X) of Ga is about 0.2 to 0.3 (Eg=about 1.15 eV). The conversion efficiency, however, starts lowering if the value (X) continues to rise (cf. H. W. Schock, MRS BULLETIN, Vol.18, No.10, p.42-44). This phenomenon is caused, probably, by the deterioration of the crystal property of the Cu(In.sub.1-X Ga.sub.X)Se.sub.2 solid solution thin film in a Ga-rich region.